1. Field of the Invention
The present invention relates to a method for generating a carrier-suppressed optical pulse train and a mode-locked semiconductor laser diode for realizing this method, for generating an optical pulse signal using the intensity modulation of a carrier-suppressed-Return to Zero format.
2. Description of Related Art
Optical communications networks are being improved to transmit information over greater distances and to provide increased capacity. A variety of formats have been proposed for the optical signals used in an optical communications system constituting an optical communications network, and a number of these optical signal formats have been commercialized. A typical commercial optical signal format is the intensity modulation format, which expresses a binary digital signal by strengthening and weakening optical intensity. This intensity modulation format can be broadly divided into two types: the NRZ (Non Return to Zero) format, in which optical intensity is maintained between consecutive “1” signals, and the RZ (Return to Zero) format, in which optical intensity returns to zero one time between consecutive “1” signals.
An optical signal of the RZ format is generated relative to an optical pulse train, in which optical signals stand in a row at regular fixed intervals on a time axis, by using an optical intensity modulator to optically modulate the individual optical pulses constituting this optical pulse train. Optically modulating individual optical pulses constituting an optical pulse train refers to generating a binary digital signal by selectively blocking and transmitting the optical pulses constituting an optical pulse train. Generating an RZ-format optical signal requires the existence of an optical pulse train beforehand, and a light source for generating this optical pulse train is indispensable.
Since an RZ-format optical signal, as described hereinabove, is a binary digital signal, which is achieved by optically modulating an optical pulse train in which optical pulses stand in a row at regular fixed intervals on a time axis, hereinafter, it is supposed that the expressions “optical pulse signal” and “optical pulse train” will be used in the following sense. That is, the expression “optical pulse signal” will only be used to signify a train of optical pulses treated as a binary digital signal, which is achieved by optically modulating a train of optical pulses standing in a row at regular fixed intervals on a time axis. Conversely, the expression “optical pulse train” will be used to designate an aggregate of optical pulses standing defect-free in a row at regular fixed intervals on a time axis.
The RZ format is one in which optical intensity returns to zero one time between consecutive “1” signals, and generally speaking the wavelength band of the light acting as the optical carrier is broader than in the NRZ format. Hereinafter, the wavelength band of the light acting as the optical carrier may also be called the optical pulse signal or the wavelength spectrum band of the optical pulse train.
Since an optical pulse denoting a bit signifying “1” already exists singly on a time axis, an RZ-format optical pulse signal will be constituted as an aggregate of optical pulses having a narrow half-width. Conversely, an NRZ-format optical pulse signal is constituted as a series of wide optical pulses between consecutive “1's” when a bit signifying “1” is expressed consecutively. Thus, the half-width of an optical pulse constituting an NRZ-format optical pulse signal is wider on average than the half-width of an optical pulse constituting an RZ-format optical pulse signal.
Therefore, the frequency band (hereinafter, may also be described as the frequency spectrum band) that an RZ-format optical pulse signal occupies is wider than the frequency spectrum band that an NRZ-format optical pulse signal occupies. In the following explanation, when there is no need to distinguish between a spectrum expressed by a frequency and a spectrum expressed by a wavelength, this frequency spectrum band may also be called simply a spectrum.
When the spectrum band is wide, firstly, there emerges a conspicuous wave form distortion effect, wherein the half-width of an optical pulse on a time axis widens due to the group-velocity dispersion of an optical fiber, which is the transmission medium of a signal, thereby restricting transmission distance. Secondly, when considering increasing capacity using a wavelength division multiplexing system, it becomes necessary to increase the wavelength difference allocated to adjacent channels in order to suppress the cross-talk between channels to which adjacent wavelengths have been allocated. In either case, an optical pulse signal having a wide spectrum band is not desirable from the standpoint of efficiently utilizing the frequency band of an optical communications network in which this optical pulse signal is being used.
Accordingly, methods for narrowing the spectrum band of an RZ-format optical pulse signal have been proposed. A typical such method is one that applies the so-called carrier-suppressed RZ format, in which the RZ format is applied to an optical pulse train, for which the phase of the optical carrier has been inverted between adjacent optical pulses on a time axis (For example, refer to A. Hirano, et al: “A novel mode-splitting detection scheme in 43-Gb/s CS- and DCS-RZ signal transmission,” IEEE J. Lightwave Technology, Vol. 20, No. 12, pp. 2029-2034, December 2002, referred to herein as Non-Patent Literature 1). Inverting the phase of the optical carrier between adjacent optical pulses on a time axis is synonymous to saying that the phase difference between adjacent optical pulses is n. Hereinafter, carrier-suppressed RZ format may also be described as CS-RZ format.
Inverting the phase of the optical carrier between adjacent optical pulses on a time axis means that the phase of the optical carrier will not be consecutive, and that a phase jump part, in which the phase of the optical carrier suddenly changes by n, exists between adjacent optical pulses. Therefore, the effect of interference that occurs between adjacent optical pulses is such that the mutual amplitudes of the adjacent optical pulses are offset. Conversely, when the phase of the optical carrier is in-phase between adjacent optical pulses on a time axis, the effect of interference that occurs between these optical pulses is such that the mutual amplitudes are added together.
The CS-RZ format makes it possible to reduce the spectrum band by about 25% compared to an ordinary RZ format for which the phase of the optical carrier between adjacent optical pulses on a time axis is the same (Refer to Non-Patent Literature 1). Thus, the CS-RZ format features outstanding resistance to waveform distortion resulting from the group-velocity dispersion of an optical fiber, and excels at making efficient use of frequency. Furthermore, the CS-RZ format suppresses waveform distortion resulting from interference between adjacent optical pulses on a time axis more than an ordinary RZ format even when the duty cycle of the optical pulse signal is high. Thus, the width on a time axis of an optical pulse constituting an optical pulse signal can be wider than with an ordinary RZ format. As a result, it is possible to reduce the spectrum band of the optical carrier. That is, using a CS-RZ-format optical pulse signal makes it possible to realize an optical communications system with outstanding long-distance transmission characteristics and frequency utilization efficiency.
Here, an optical pulse duty cycle refers to the ratio of the half-widths of pertinent optical pulses relative to the interval of optical pulses lined up adjacently on a time axis (the time width per bit, and may also be called the time slot). Therefore, when the duty cycle is high, it means that the half-width of an optical pulse is wide relative to the time slot. That is, the duty cycle will become high when the time slot is fixed, and the optical pulse half-width widens, or when the optical pulse half-width is fixed, and the time slot narrows.
The following four methods were proposed in the past as methods for generating a CS optical pulse train, which is deemed necessary for generating an optical pulse signal of the CS-RZ format.
The first method is one that uses a Mach-Zehnder interferometer-type LiNbO3 optical intensity modulator (For example, refer to Non-Patent Literature 1). Hereinafter, a LiNbO3 optical intensity modulator may also be described as a LN optical intensity modulator. This method will be explained using an example of the generation of a CS optical pulse train for which the repetitive frequency is 40 GHz. First, a continuance wave (CW) light generated from a CW light source is inputted to an LN optical intensity modulator. Then, if the DC bias level of the LN optical intensity modulator is set to the minimum transmittance voltage value, the repetitive frequency is 20 GHz, and the LN optical intensity modulator is modulated at an electric modulation signal (most often a sine wave) in which the difference of the maximum and minimum voltages (the peak-to-peak voltage, which hereinafter may also be described as Vpp) is two times the half-wavelength voltage Vπ, a CS optical pulse train with repetitive frequency of 40 GHz is outputted from the LN optical intensity modulator.
According to the first method, changes in optical pulse characteristics are small even when the wavelength of the CW light source changes, thereby making it possible to provide a high-performance, wavelength-variable CS optical pulse train-generating light source. This is due to the small wavelength dependence of the optical modulation characteristics of the LN optical intensity modulator. Further, the first method also has the advantage of making it easy to change the repetitive frequency.
A second method is one that utilizes a mode-locked semiconductor laser diode into which has been integrated a chirped grating, and uses chirped grating dispersion to make the laser oscillate in two modes (For example, refer to K. Sato, et al: “Dual mode operation of semiconductor mode-locked lasers for anti-phase pulse generation,” Technical Digest of OFC 2000, paper ThW3-1, 2000, referred to herein as Non-Patent Literature 2). For the sake of expediting the explanation, three longitudinal modes in the vicinity of the Bragg wavelength of the chirped grating are considered here. The frequencies of these three modes, from the low frequency side, are treated as fm−1, fm, and fm+1. Then, chirped grating dispersion is utilized, and the frequency difference between the m−1 order and m order longitudinal modes (fm−fm−1) and the frequency difference between the m order and m+1 order longitudinal modes (fm+1−fm) are treated as values that are so different that frequency pulling resulting from mode-locked operation does not occur. Here, m is an integer.
When mode locking is imposed on this mode-locked semiconductor laser diode by applying modulation at the frequency (fm+1−fm), the m−1 order mode does not go into mode lock, and does not perform mode-locked oscillation because frequency pulling does not occur. That is, this laser oscillates in two modes, the m order mode and the m+1 order mode. The dual mode oscillation state is the most basic CS optical pulse train generating state. Therefore, this method enables the generation of a CS optical pulse train.
The second method described hereinabove is advantageous in that CS optical pulse train generation can be achieved using a single element, enabling the device for realizing this method to be made smaller and less expensive.
A third method is a more generalized version of the second method described above. That is, it is a method that provides two single-longitudinal-mode oscillation lasers having different wavelengths, and generates a CS optical pulse train of a repetitive frequency equivalent to the wavelength difference of the two lasers by combining the outputs of the two lasers.
The third method is advantageous in that it is possible to change the wavelength and repetitive frequency by changing the wavelengths of the two single-longitudinal-mode oscillation lasers.
A fourth method is one that uses an optical pulse light source and an optical delay interferometer. This method will be explained using an example of the generation of a CS optical pulse train for which the repetitive frequency is 40 GHz. First, an optical pulse light source is provided for generating, at a repetitive frequency of 20 GHz, an ordinary optical pulse train in which the phases of the optical carriers between adjacent optical pulses are uniform. Next, this optical pulse train is divided in two. Using retarding optics, a time delay of 25 ps is applied to one side of this divided optical pulse train at the same time as a phase difference of n is applied as the optical carrier. Thereafter, a CS optical pulse train with a repetitive frequency of 40 GHz is produced by coupling the two sides of the divided optical pulse train. Optical fiber-type retarding optics can be used as the optical divider/coupler and retarding optics, and as disclosed by H. Murai, et al in “EA modulator-based optical multiplexing/demultiplexing techniques for 160 Gbit/s OTDM signal transmission,” IEICE Trans. Electron., vol. E88-C, No. 3, pp. 309-318, March 2005, referred to herein as Non-Patent Literature 3, retarding optics of a constitution, which combines a half mirror and spatial optics, can also be used.
However, because the first method requires a CW light source in addition to the LN optical intensity modulator, a device for realizing the first method will be large. Further, the half-wavelength voltage Vπ of a common LN optical intensity modulator is between 5V and 10V, but since the modulation voltage Vpp deemed necessary is 2Vπ, the required modulation voltage Vpp becomes between 10V and 20V. This makes the impedance of the LN optical intensity modulator 50Ω, which, if converted to electrical power, works out to a large value of between 24 dBm and 30 dBm, meaning that the first method requires high power consumption.
In the second method, as a rule, only a sine wave optical pulse train can be achieved. That is, in the second method, it is not possible to set a flexible pulse width corresponding to system specifications.
In the third method, too, as a rule, only a sine wave optical pulse train can be achieved. Further, in the third method, oscillation must be carried out by phase-locking two lasers, and a control device is needed to realize this phase locking. As a result, a device for realizing the third method is large and expensive.
To realize the fourth method described above requires a light source for generating an ordinary optical pulse train of a frequency that is one-half that of the repetitive frequency of a CS optical pulse train (20 GHz in the example described hereinabove). Here, ordinary optical pulse train refers to an optical pulse train in which the phases of the optical pulses constituting this optical pulse train are equal. In the fourth method, if optical carrier phase control is taken into account, it is necessary to use an optical delay interferometer to execute high-precision optical delay control corresponding to several micrometers in terms of geometric length. That is, the constitution of a device for realizing the fourth method is complex, and requires a high-precision control circuit, making it large and expensive.
Accordingly, an object of the present invention is to provide a CS optical pulse generation method, and more particularly, a CS optical pulse generation method, which is capable of changing the half-width of an optical pulse constituting this CS optical pulse train, and which is small in scale and capable of being carried out using little power.
Further, another object of the present invention is to provide a mode-locked semiconductor laser diode for realizing this method.
Furthermore, in the field of technology for operating a semiconductor laser diode in a mode-locked state, a semiconductor laser diode, which integrally incorporates an optical modulator or other such device that is deemed necessary for realizing mode-locked operation, is used. For this reason, mode-locked is a term signifying an essential laser operating configuration, and a semiconductor laser diode that is designed and manufactured on the premise of carrying out mode-locked operation may be called a mode-locked semiconductor laser diode. Therefore, since the distributed Bragg reflector semiconductor laser, Fabry-Perot external-cavity type semiconductor laser, and ring resonator-type semiconductor laser explained hereinbelow have been designed and manufactured on the premise of carrying out mode-locked operation, they are all mode-locked semiconductor laser diodes.